An active, partially fractionated cell-free protein synthesizing system has been established from rabbit lung. Collagen in lung is synthesized on the class of large polysomes associated with the endoplasmic reticulum. During lung development there are significant changes in the relative proportion of lung polysomes that synthesize collagen suggesting that at least some of the control of lung collagen synthesis may be due to changes in lung collagen gene expression or to changes in the number of cells that synthesize collagen.